This invention relates to a ferrule type fitting for large diameter tubes.
Ferrule type fittings have been known to the art for many years. U.S. Pat. No. 3,103,373 to Lennon et al. is an example of a commercial tube fitting in widespread use today. Tube fittings of the type shown in U.S. Pat. No. 3,103,373, however, have heretofore been used in applications where the tube is 1 inch or less in diameter. The design of tube fittings over 1 inch has been found difficult to successfully achieve because of an apparent need for understanding of design factors not apparent in small diameter fittings.
Experimentation with large diameter tubes and fittings of the type shown in the Lennon U.S. Pat. No. 3,103,373 has shown that where a leak was found to exist it was most often caused by the interaction of the back ferrule with the wave induced in the tube by the front ferrule on make-up. Specifically, in such cases the deformation of the tube caused by the front ferrule interfered with effective sealing and gripping of the tube by the back ferrule, causing the overall sealing and gripping action of the fitting to be impaired. The problem became particularly acute in connection with thin wall tubing.
As the result of a close examination of the problem, it has been found that a physical relationship exists between the axial length of the wave or indentation induced in a tube by a ferrule and the inside and outside diameters of the tube. This relationship is such that with all the conditions being equal, a ferrule will induce a longer wave in a thin wall tube than in a standard or heavy wall tube. Further, it has been found that the axial length of the wave so induced will vary within limits from a very small percentage of O.D. (where O.D. is the outside diametr of the tube) for heavy wall tubes to one-half of O.D. for thin wall tubes. That is, when the nose of a ferrule is deformed inwardly into engagement with a tube, it forms a wave or indentation in the tube having an effective axial length equal to one-half the outside diameter of the tube for very thin wall tubes and less for heavy wall tubes.
Having determined that the axial length of the wave or indentation induced in a tube by a ferrule is related to the tube O.D. and the outside and inside diameter ratio of the tube, it became possible to predict that:
a. as the outside diameter of the tube was increased, and the ratio of the outside diameter to inside diameter remained constant, the axial length of the wave induced in the tube by a ferrule would correspondingly increase; PA1 b. as the wall thickness of the tube was decreased, with the outside diameter of the tube remaining constant, the axial length of the wave induced in the tube by a ferrule would correspondingly increase.
For a clear understanding of the relationship of the length of the wave in the tube with respect to the tube O.D. and the ratio of tube O.D. to tube I.D. the effect of the laws governing this action are given in paragraphs (a) and (b) above.
It must also be recognized that the ratio of tube O.D. to tube I.D. (O.D./I.D.) approaches a limit of 1 when I.D. approaches O.D. and reaches infinity when I.D. diminishes to zero.
In order that a fitting be practical it must function against a range of tube O.D. to I.D. ratios with ferrule spacing being largest when I.D. approaches O.D. for a ratio of 1 for an infinitely thin wall.
Therefore thin wall applications impose a design consideration in regard to front ferrule length and ferrule spacing to avoid wave interaction.
The nature of the laws governing wave action have as one of its factors the O.D. to I.D. ratio. Because the O.D. to I.D. ratio cannot be less than 1 there is an optimum ferrule spacing that need not be exceeded. Shorter spacing will secure useful fitting performance dropping from optimum to lesser and lesser reliability as spacing is decreased.
When designing for a given size tubing and assuming thin wall tubing approaching a O.D. to I.D. ratio of 1, ferrule spacing is then governed only by the single variable of tube diameter resulting in a simple ratio relationship to tube O.D. for ductile materials.
The importance of understanding the relationship to tube dimensions of the axial length of the wave induced in the tube by the ferrule will be appreciated when it is understood that effective sealing and gripping of the tube by the ferrule is, in itself, dependent upon the nature of the wave thus induced.
In large diameter fitting designs where a leak occurred it was seen that the axial length of the front ferrule of the fitting was relatively short as compared to the axial length of the wave induced in the thin wall tube by the front ferrule. As a consequence, the wave induced in the tube by the front ferrule extended into the vicinity of the nose portion of the back ferrule, with the result that effective sealing of the front ferrule and gripping of the tube by the back ferrule was impeded. It was noted that the wave in the tube induced by the back ferrule interacted with the wave in the tube produced by the front ferrule tending to form one wave in the tube instead of two.
The result was to impair the grip of the back ferrule which is greatest when it produces a wave independent of the front ferrule wave. Under the above condition where the front ferrule was short the back ferrule tended to nose under the front ferrule thus pulling the tube away from the front ferrule to the detriment of the sealing function of the front ferrule.
The invention in its broadest sense is directed to a multiple ferrule-type fitting adapted for use with large diameter tubes and wherein the axial length of one ferrule has a minimum value which is a function of the outside diameter of the tube such that upon make-up of the fitting the wave induced in the tube by that ferrule does not encounter the nose portion of any other ferrule(s).